1. Field of the Invention
The present invention relates to a wireless access system, and more particularly, to a method and apparatus for reducing interference between systems, when a plurality of wireless access systems are coexistent.
2. Discussion of the Related Art
A typical frame structure used for a wireless access system will be described.
FIG. 1 illustrates a frame structure used in a broadband wireless access system (e.g. Institute of Electrical and Electronics Engineers (IEEE) 802.16e)).
Referring to FIG. 1, the horizontal axis of a frame represents the numbers of time units, specifically Orthogonal Frequency Division Multiple Access (OFDMA) symbols and the vertical axis of the frame represents the numbers of frequency units, specifically the logical numbers of subchannels. One frame is divided into data sequence channels each having a predetermined time duration according to its physical characteristics. That is, a frame is divided into a DownLink (DL) subframe and an UpLink (UL) subframe.
The DL subframe may include a preamble, a Frame Control Header (FCH), a DL-MAP, a UL-MAP, and one or more data bursts. The UL subframe may include one or more UL data bursts and a ranging subchannel.
Referring to FIG. 1, the preamble is a predetermined sequence positioned in the first symbol of each frame, for use in acquiring synchronization to a Base Station (BS) and channel estimation at a Mobile Station (MS). The FCH provides channel allocation and channel coding information associated with the DL-MAP. The DL-MAP and the UL-MAP are Medium Access Control (MAC) messages by which DL and UL channel assignments are transmitted to MSs. The data bursts are data units that a BS transmits to MSs or MSs transmit to the BS.
A Downlink Channel Descriptor (DCD) in the frame is a MAC message that describes the physical characteristics of a DL channel, and an Uplink Channel Descriptor (UCD) is a MAC message that describes the physical characteristics of a UL channel.
On a UL, an MS acquires synchronization to a BS by detecting a preamble transmitted from the BS. Subsequently, the MS may decode a DL-MAP using information acquired from an FCH. The BS may transmit scheduling information for DL or UL resource allocation to the MS in every frame, for example, every 5 ms by a DL-MAP/UL-MAP message.